Signal transduction affecting cell growth and differentiation is regulated in part by phosphorylation of various cellular proteins. Protein tyrosine kinases are enzymes that catalyze this process. Receptor protein tyrosine kinases are believed to direct cellular growth via ligand-stimulated tyrosine phosphorylation of intracellular substrates. Growth factor receptor protein tyrosine kinases of the class I subfamily include the 170 kDa epidermal growth factor receptor (EGFR) encoded by the erbB1 gene. erbB1 has been causally implicated in human malignancy. In particular, increased expression of this gene has been observed in more aggressive carcinomas of the breast, bladder, lung and stomach. The second member of the class I subfamily, p185.sup.neu, was originally identified as the product of the transforming gene from neuroblastomas of chemically treated rats. The neu gene (also called erbB2 and HER2) encodes a 185 kDa receptor protein tyrosine kinase. Amplification and/or overexpression of the human HER2 gene correlates with a poor prognosis in breast and ovarian cancers (Slamon et al., (1987) Science 235:177-182; and Slamon et al., (1989) Science 244:707-712). Overexpression of BER2 has been correlated with other carcinomas including carcinomas of the stomach, endometrium, salivary gland, lung, kidney, colon and bladder. A further related gene, called erbB3 or BER3, has also been described (Kraus et al., (1989) Proc. Natl. Acad. Sci. USA 86:9193-9197). Kraus et al. (1989) discovered that markedly elevated levels of erbB3 mRNA were present in certain human mammary tumor cell lines indicating that erbB3, like erbB1 and erbB2, may play a role in human malignancies. The erbB3 gene has been found to be overexpressed in breast (Lemoine et al. (1992) Br. J. Cancer 66:1116-1121), gastrointestinal (Poller et al (1992) J. Pathol. 168:275-280, Rajkumer et al. (1993) J. Pathol. 170:271-278, and Sanidas et al. (1993) Int. J. Cancer 54:935-940, and pancreatic cancers (Lemoine et al. (1992) J. Pathol. 168:269-273, and Friess et al. (1995) Clinical Cancer Research 1:1413-1420).
The class I subfamily of growth factor receptor protein tyrosine kinases has been further extended to include the HER4/Erb4 receptor (EP Pat Appln No 599,274; Plowman et al. (1993) Proc. Natl. Acad. Sci. USA 90:1746-1750; and Plowman et al. (1993) Nature 366:473-475. Plowman et al. found that increased HER4 expression closely correlated with certain carcinomas of epithelial origin, including breast adenocarcinomas. Diagnostic methods for detection of human neoplastic conditions (especially breast cancers) which evaluate HER4 expression are described in EP Pat Appln No. 599,274.
The quest for the activator of the HER2 oncogene has lead to the discovery of a family of polypeptides, collectively called neuregulins (NRG1). These proteins appear to result from alternate splicing of a single gene which was mapped to the short arm of human chromosome 8 by Orr-Urtreger et al. (1993) Proc. Natl. Acad. Sci. USA 90:1867-1871.
Holmes et al. isolated and cloned a family of polypeptide activators for the HER2 receptor which they called heregulin-.alpha. (HRG-.alpha.), heregulin-.beta.1 (HRG-.beta.1), heregulin-.beta.2 (HRG-.beta.2), heregulin-.beta.2-like (HRG-.beta.2-like), and heregulin-.beta.3 (HRG-.beta.3). See Holmes et al. (1992) Science 256:1205-1210; WO 92/20798; and U.S. Pat. No. 5,367,060. The 45 kDa polypeptide, HRG-.alpha., was purified from the conditioned medium of the MDA-MB-231 human breast cancer cell line. These researchers demonstrated the ability of the purified heregulin polypeptides to activate tyrosine phosphorylation of the HER2 receptor in MCF7 breast tumor cells. Furthermore, the mitogenic activity of the heregulin polypeptides on SK-BR-3 cells (which express high levels of the HER2 receptor) was illustrated. Like other growth factors which belong to the EGF family, soluble HRG polypeptides appear to be derived from a membrane bound precursor (called pro-HRG) which is proteolytically processed to release the 45 kDa soluble form. These pro-HRGs lack a N-terminal signal peptide.
While heregulins are substantially identical in the first 213 amino acid residues, they are classified into two major types, .alpha. and .beta., based on two variant EGF-like domains which differ in their C-terminal portions. Nevertheless, these EGF-like domains are identical in the spacing of six cysteine residues contained therein. Based on an amino acid sequence comparison, Holmes et al. found that between the first and sixth cysteines in the EGF-like domain, HRGs were 45% similar to heparin-binding EGF-like growth factor (HB-EGF), 35% identical to amphiregulin (AR), 32% identical to TGF-.alpha., and 27% identical to EGF.
The 44 kDa neu differentiation factor (NDF), which is the rat equivalent of human HRG, was first described by Peles et al. (1992) Cell 69:205-216; and Wen et al. (1992) Cell 69:559-572. Like the HRG polypeptides, NDF has an immunoglobulin (Ig) homology domain followed by an EGF-like domain and lacks a N-terminal signal peptide. Subsequently, Wen et al. (1994) Mol. Cell. Biol. 14(3). 1909-1919 carried out cloning experiments to extend the family of NDFs. This work revealed six distinct fibroblastic pro-NDFs. Adopting the nomenclature of Holmes et al., the NDFs are classified as either .alpha. or .beta. polypeptides based on the sequences of the EGF-like domains. Isoforms 1 to 4 are characterized on the basis of the region between the EGF-like domain and transmembrane domain. Also, isoforms a, b and c are described which have variable length cytoplasmic domains. These researchers conclude that different NDF isoforms are generated by alternative splicing and perform distinct tissue-specific functions. See also EP 505 148; WO 93/22424; and WO 94/28133 concerning NDF.
Falls et al. (1993) Cell 72:801-815 describe another member of the heregulin family which they call acetylcholine receptor inducing activity (ARIA) polypeptide. The chicken-derived ARIA polypeptide stimulates synthesis of muscle acetylcholine receptors. See also WO 94/08007. ARIA is a .beta.-type heregulin and lacks the entire spacer region rich in glycosylation sites between the Ig-like domain and EGF-like domain of HRG.alpha., and HRGP.beta.1-.beta.3.
Marchionni et al. identified several bovine-derived proteins which they call glial growth factors (GGFs) (Marchionni et al. (1993) Nature 362:312-318). These GGFs share the Ig-like domain and EGF-like domain with the other heregulin proteins described above, but also have an amino-terminal kringle domain. GGFs generally do not have the complete glycosylated spacer region between the Ig-like domain and EGF-like domain. Only one of the GGFs, GGFII, possessed a N-terminal signal peptide. See also WO 92/18627; WO 94/00140; WO 94/04560; WO 94/26298; and WO 95/32724 which refer to GGFs and uses thereof.
Ho et al. in (1995) J. Biol. Chem. 270(4):14523-14532 describe another member of the heregulin family called sensory and motor neuron-derived factor (SMDF). This protein has an EGF-like domain characteristic of all other heregulin polypeptides but a distinct N-terminal domain. The major structural difference between SMDF and the other heregulin polypeptides is the lack in SMDF of the Ig-like domain and the "glyco" spacer characteristic of all the other heregulin polypeptides. Another feature of SMDF is the presence of two stretches of hydrophobic amino acids near the N-terminus.
While the heregulin polypeptides were first identified based on their ability to activate the HER2 receptor (see Holmes et al., supra), it was discovered that certain ovarian cells expressing neu and neu-transfected fibroblasts did not bind or crosslink to NDF, nor did they respond to NDF to undergo tyrosine phosphorylation (Peles et al. (1993) EMBO J. 12:961-971). This indicated that another cellular component was necessary for conferring full heregulin responsiveness. Carraway et al. subsequently demonstrated that .sup.125 I-rHRG.beta.1.sub.177-244 bound to NIH-3T3 fibroblasts stably transfected with bovine erbB3 but not to non-transfected parental cells. Accordingly, they conclude that ErbB3 is a receptor for HRG and mediates phosphorylation of intrinsic tyrosine residues as well as phosphorylation of ErbB2 receptor in cells which express both receptors. Caraway et al. (1994) J. Biol. Chem. 269(19):14303-14306. Sliwkowski et al. (1994) J. Biol. Chem. 269(20):14661-14665 found that cells transfected with HER3 alone show low affinities for heregulin, whereas cells transfected with both HER2 and HER3 show higher affinities.
This observation correlates with the "receptor cross-talking" described previously by Kokai et al., Cell 58:287-292 (1989); Stem et al. (1988) EMBO J. 7:995-1001; and King et al., 4:13-18 (1989). These researchers found that binding of EGF to the EGFR resulted in activation of the EGFR kinase domain and cross-phosphorylation of p185.sup.HER2. This is believed to be a result of ligand-induced receptor heterodimerization and the concomitant cross-phosphorylation of the receptors within the heterodimer (Wada et al. (1990) Cell 61:1339-1347).
Plowman and his colleagues have similarly studied p185.sup.HER4 /p185.sup.HER2 activation. They expressed p185.sup.HER2 alone, p185.sup.HER4 alone, or the two receptors together in human T lymphocytes and demonstrated that heregulin is capable of stimulating tyrosine phosphorylation of p185.sup.HER4, but could only stimulate p185.sup.HER2 phosphorylation in cells expressing both receptors. Plowman et al., Nature 336:473-475 (1993). Thus, heregulin is the only known example of a member of the EGF growth factor family that can interact with several receptors. Carraway and Cantley (1994) Cell 78:5-8.
The biological role of heregulin has been investigated by several groups. For example, Falls et al., (discussed above) found that ARIA plays a role in myotube differentiation, namely affecting the synthesis and concentration of neurotransmitter receptors in the postsynaptic muscle cells of motor neurons. Corfas and Fischbach demonstrated that ARIA also increases the number of sodium channels in chick muscle. Corfas and Fischbach (1993) J. Neuroscience 13(5): 2118-2125. It has also been shown that GGFII is mitogenic for subconfluent quiescent human myoblasts and that differentiation of clonal human myoblasts in the continuous presence of GGFII results in greater numbers of myotubes after six days of differentiation (Sklar et al. (1994) J. Cell Biochem., Abst. W462, 18D, 540). See also WO 94/26298 published Nov. 24, 1994.
Holmes et al., supra, found that HRG exerted a mitogenic effect on mammary cell lines (such as SK-BR-3 and MCF-7). The mitogenic activity of GGFs on Schwann cells has also been reported. See, e.g., Brockes et al. (1980) J. Biol. Chem. 255(18):8374-8377; Lemke and Brockes (1984) J. Neurosci. 4:75-83; Brockes et al. (1984) J. Neuroscience 4(1):75-83; Brockes et al. (1986) Ann. Neurol. 20(3):317-322; Brockes, J. (1987) Methods in Enzym. 147:217-225 and Marchionni et al., supra. Schwann cells constitute important glial cells which provide myelin sheathing around the axons of neurons, thereby forming individual nerve fibers. Thus, it is apparent that Schwann cells play an important role in the development, function and regeneration of peripheral nerves. The implications of this from a therapeutic standpoint have been addressed by Levi et al. (1994) J. Neuroscience 14(3):1309-1319. Levi et al. discuss the potential for construction of a cellular prosthesis comprising human Schwann cells which could be transplanted into areas of damaged spinal cord. Methods for culturing Schwann cells ex vivo have been described. See WO 94/00140 and Li et al. (1996) J. Neuroscience 16(6):2012-2019.
Pinkas-Kramarski et al. found that NDF seems to be expressed in neurons and glial cells in embryonic and adult rat brain and primary cultures of rat brain cells, and suggested that it may act as a survival and maturation factor for astrocytes (Pinkas-Krarnarski et al. (1994) PNAS, USA 91:9387-9391). Meyer and Birchmeier (1994) PNAS, USA 91:1064-1068 analyzed expression of heregulin during mouse embryogenesis and in the perinatal animal using in situ hybridization and RNase protection experiments. These authors conclude that, based on expression of this molecule, heregulin plays a role in vivo as a mesenchymal and neuronal factor. Also, their findings imply that heregulin functions in the development of epithelia. Similarly, Danilenko et al. (1994) Abstract 3101, FASEB 8(4-5):A535, found that the interaction of NDF and the HER2 receptor is important in directing epidermal migration and differentiation during wound repair.
Although NRG1 was initially proposed to be the ligand for the receptor tyrosine kinase ErbB2, further studies have demonstrated that activation of ErbB2 frequently occurred as a result of NRG1 binding to ErbB3 (Sliwkowski, M. X., et al. (1994) J. Biol. Chem. 169:14661-14665) or ErbB4 (Plowman, G. D. et al. (1993) Nature 366:473-475; and Carraway, K. L. and Cantley, L. C. (1994) Cell 78:5-8) receptors. Recent studies have begun to highlight the roles of NRG1, ErbB2 receptor and ErbB4 receptor in the development of the heart. Mice lacking ErbB4 receptor, ErbB2 receptor or NRG1 die during mid-embryogenesis (embryonic day 10.5) from the aborted development of myocardial trabeculae in the ventricle (Meyer & Birchmeier (1995) Nature 378:386-90; Gassmann et al. (1995) Nature 378:390-4; and Lee et al. (1995) Nature 378:394-8). These results are consistent with the view that NRG1, expressed in the endocardium, is an important ligand required for the activation of ErbB2 and ErbB4 receptors in the myocardium.
These same studies suggest that NRG1 and ErbB2 receptor may play a different role than ErbB4 receptor in the development of the hind brain. NRG1 is expressed in the neuroepithelium and cells arising from rhombomeres 2, 4 and 6, while ErbB4 receptor is expressed in rhombomeres 3 and 5. NRG1 and ErbB2 receptor knockout mice exhibit a loss of cells and axons of the cranial sensory ganglia. In contrast, ErbB4 receptor deficient mice do not exhibit a loss of cellularity in the cranial ganglia. Rather, the organization, spacing and pattern of innervation of these ganglia to and from the central nervous system is disrupted (Gassmann et al., supra). One possible reason for this difference in hindbrain phenotypes of NRG1 and ErbB4 receptor knockout mice is that additional ligand(s) distinct from NRG1 may be recognized by ErbB4 in the CNS (Gassmann et al., supra).